In a wide variety of applications, an opening between two different environments must be sealed, for example, to maintain a pressure differential or to prevent materials from one environment from entering the other (hereinafter "an exclusion seal"). Such seals can be dynamic, allowing movement of the sealed parts, or static. An example of the former is found in rotary feedthroughs, which consist of a rotating shaft surrounded by a housing. These devices are used to transfer rotary motion from an environment at a selected pressure (such as atmospheric pressure) to a vacuum, to or from a hazardous environment, or to or from a "clean" environment (such as is required for semiconductor or disk drive production). Seals allowing limited axial motion of a shaft in a housing have also been described.
Solid sealants are well known and include rubber, VITON, TEFLON, or polytetrafluoroethylene composite polymer. Although such solid sealants can be effective for static applications, these materials are unsuitable for many dynamic applications. Prior art solid O-rings used to seal a rotating shaft, for example, wear relatively rapidly and shed particles that contaminate the environments on either side of the seal. Other prior art sealants include gases, oils, greases, and "ferrofluids."
Ferrofluids are magnetic liquids that are used as sealants in magnetic liquid-sealed rotary feedthroughs. Such feedthroughs typically consist of a shaft held in a housing by a pair of bearings on each end of the housing. In between the bearings, the housing also holds an arrangement of an annular magnet sandwiched between two annular pole piece elements, which surrounds the shaft. The pole piece elements are each designed to form a very small gap with the rotating shaft and to concentrate the magnetic field in this region. The magnetic field retains ferrofluid in the gap, forming a liquid "O-ring."
A wide variety of modifications of this basic configuration have been patented, among them designs aimed at providing stronger seals, increasing seal life and convenience of use, and decreasing seal size and cost. U.S. Pat. No. 4,445,696 (issued to Raj et al., May 1, 1984) discloses a device designed to provide a nonbursting seal for high vacuum applications. U.S. Pat. No. 4,506,895 (issued to Raj, Mar. 26, 1985) teaches a magnetic seal that self-activates when a feedthrough is assembled. A composite ferrofluid bearing and seal apparatus incorporating a fluid storage cavity is described in U.S. Pat. No. 4,630,943 (issued to Stahl et al., Dec. 23, 1986). A plurality of magnets disposed in series (a multi-stage sealing device) to increase the pressure differential capability of the seals is disclosed in U.S. Pat. No. 4,605,233 (issued to Sato on Aug. 12, 1986).
The ferrofluid in such devices generally consists of a suspension of small ferromagnetic particles (on the order of 100 .ANG. in diameter) in a liquid base. The particles are typically coated with a surfactant to reduce clumping caused by magnetic attraction.
A wide variety of ferrofluid compositions have been developed, however, all contain significant amounts of liquid, which has a number of drawbacks. First, liquid compositions are subject to "outgassing," in which one or more components volatilizes and contaminates one or both of the sealed environments. Second, the components of prior art ferrofluids tend to separate out, reducing seal effectiveness, especially in very strong magnetic fields. Third, seal effectiveness is also compromised by the clumping of the magnetic particles in the ferrofluid, which can result in an uneven particle distribution over the sealing area. Because of these problems, the pressure differential capacity of prior art ferrofluid seals has been limited to 5 psi/stage. Multi-stage ferrofluid seals have been employed to provide vacuum chamber pressures as low as 5.times.10.sup.-9 Torr; however, lower pressures are difficult to maintain due to outgassing from the ferrofluid seal material.
A final disadvantage the prior art ferrofluids is the "wetting tendency" of the liquid in these compositions. This tendency of liquids to stick to surfaces results in fluid loss from the seal, which, in turn, reduces seal life and causes contamination problems. Such problems are particularly severe in applications where the seal encloses a shaft that moves axially. Magnetic liquid-sealed axial feedthroughs are disclosed in U.S. Pat. No. 4,309,040 (issued to Peirrat, Jan. 5, 1982) and U.S. Pat. No. 4,502,700 (issued to Gowda et al., Mar. 5, 1985). The former teaches the use of a pressure ring and scraper ring to prevent escape of the ferrofluid, whereas the latter describes the use an annular collector magnet and a rod wiper to contain the ferrofluid. While these designs reduce fluid loss due to wetting, neither design eliminates the problem, which limits the utility of such seals.
Attempts to circumvent this problem have employed a bellows assembly or magnetic coupling and/or levitation strategies. However, these approaches are costly, and devices employing magnetic coupling tend to be bulky. A magnetic liquid-sealed axial feedthrough is less expensive and more compact, and thus would be preferred if the problems associated with liquid sealants could be avoided.